Fast DC charging stations (DC: direct current) for electric cars which are available at present usually operate with a charging voltage of 400 volts. The charging power of charging stations of this kind lies in the region of 50 kilowatts. In order to allow even faster charging processes and therefore to further improve the user friendliness, in particular from the point of view of the ratio of driving time to charging time, higher battery voltages, which lie far above 400 V of the typical fast DC charging posts, are increasingly being used in drive systems of electrically driven vehicles. One example which may be mentioned here is the turbocharging concept by Porsche in which the voltage level during the charging process is 800 volts. Doubling the voltage level with the current intensity remaining the same and therefore with the same electrical loading of the charging pin on the charging cable allows the charging time to be halved.
However, for the user of a modern electric vehicle in which the 800-volt high-power charging technique is used, this can be problematical since, at present, the majority of fast charging stations still operate at a voltage level of 400 volts and cannot provide 800 volts. In order to nevertheless be able to charge an electric vehicle with the 800-volt high-power charging technique at existing 400 V fast charging posts, DC voltage conversion has to be performed, by means of which the charging voltage of the charging post is increased from, for example, 400 V to 800 V for the HV vehicle battery.
This DC/DC conversion which is required for raising the voltage level can be performed, for example, by a dedicated DC/DC converter which, however, on account of the high power required, is expensive and occupies a large amount of installation space, and therefore does not constitute a practical solution to the problem. In addition, a DC/DC converter of this kind increases the weight of the vehicle to a significant extent, as a result of which the range is adversely affected.
As an alternative, the inverter of the vehicle, which is also called the drive inverter, can be used for a purpose other than that intended. The functioning of a step-up converter (also called boost converter) can be replicated with the aid of the phase inductance of the electrical machine and by suitably driving the semiconductor elements of the drive inverter. The step-up converter then converts the relatively low voltage of the charging post to the higher voltage of the high-voltage battery (called HV battery in the text which follows) of the vehicle, wherein the conversion is performed by means of the phase inductor of the electrical machine (from phase to star point). A charging process which takes place by means of the drive inverter of the electric car has the advantage that an 800 V HV battery can be charged at any conventional charging post by said charging process, without the use of additional charging electronics (such as, for example, a dedicated DC/DC converter).
The charging process of electric vehicles at charging stations can be standardized, for example, in accordance with the mandates of the international standard IEC 61851 (International Electrotechnical Commission), which is incorporated by reference herein, wherein this series of standards is applicable in a number of European countries. According to this standard, the charging station is at zero voltage until an electric vehicle is connected, and the charging current is switched on only after a staffing or initialization sequence has been successfully run. In addition to checking the insulation of the vehicle, voltage synchronization is a primary objective of the starting sequence. This is understood to mean matching the charging voltage of the charging post to the voltage of the HV battery (within a predefined tolerance). During the charging process of an HV battery which takes place by means of the drive inverter, the charging station is not connected directly to the battery of the vehicle as is usually the case, but rather is connected to one of the two poles at the star point of the electrical machine. As a result, the drive inverter and the electrical machine are connected between the charging station and the HV battery, so that this topology does not permit voltage synchronization in accordance with the mandates of IEC 61851.